The present invention relates generally to a gas turbine blade component comprised of two or more components made from different materials, and more particularly to a process used in the construction of a lightweight jet engine fan blade.
Gas turbine blades include, but are not limited to, gas turbine power generation equipment and gas turbine aircraft engines. A gas turbine includes a core engine having a compressor to compress the air flow entering the core engine, a combustor in which a mixture of fuel and the compressed air is burned to generate a propulsive gas flow, and a turbine which is rotated by the propulsive gas flow and which is connected by a larger diameter shaft to drive the compressor. A typical front fan gas turbine aircraft engine includes a high pressure turbine and a low pressure turbine, located aft of the high pressure turbine, which is connected by a smaller diameter coaxial shaft to drive a front fan, located fore of the compressor. The compressor may optionally include a low pressure compressor and a high pressure compressor, both located aft of the fan. The low pressure compressor is sometimes called a booster compressor or simply a booster.
The fan, the compressor blades and turbine blades have airfoils each include a airfoil portion attached to a shank portion and are attached to a rotating disc. Stator vanes are stationary airfoils which are attached to a non-rotating stator casing or engine frame. Stator vanes direct the flow of air from the rotating blades. Typically, there are alternating circumferential rows of rotor blades extending outwardly from the discs and stator vanes extending inwardly from the casings. When present, a first and/or last row of stator vanes (also called inlet and outlet guide vanes) may have their radially-inward ends also attached to a non-rotating gas turbine stator casing.
Conventional airfoil designs used in the compressor section at the engine typically have airfoil portions that are made entirely of metal, such as titanium or titanium alloys, or are made entirely of composite. A xe2x80x9ccompositexe2x80x9d is defined to be a material having any (metal or non-metal) fiber filament embedded in any (metal or non-metal) matrix, but the term xe2x80x9ccompositexe2x80x9d does not include a metal fiber embedded in a metal matrix. The term xe2x80x9cmetalxe2x80x9d includes alloys such as titanium alloy Ti 6-4. An example of a composite is a material having graphite filaments embedded in an epoxy resin.
Fan blades are large and are fore of the compressor, while compressor blades in the front of the engine are large and become progressively smaller as the combustor portion of the engine is approached. Conversely, turbine blades are small and become progressively larger with departure from the combustor portion of the engine. Fan blades are usually the largest blades in the engine.
The all-metal blades, including costly wide-chord hollow blades used as fan blades, are heavier in weight which results in lower fuel performance and require sturdier blade attachments, while the lighter all-composite blades are more susceptible to damage from ingestion events also known as foreign object damage (xe2x80x9cF.O.Dxe2x80x9d). Known to the art are hybrid blades including a composite blade having an airfoil shape which is covered by a surface cladding (with only the blade tip and the leading and trailing edge portions of the surface cladding comprising a metal) for erosion and F.O.D. The fan blades typically are the largest (and therefore the heaviest) blades in a gas turbine aircraft engine, and the front fan blades are usually the first to undergo F.O.D.
Various designs are under construction for gas turbine blades having reduced weight for use as gas turbine fan blades that are comprised of a combination of monolithic metal and non-metal materials and have the capability to resist F.O.D. Some of these designs include lightweight inserts molded into cavities of metal blades. The cavities are regions of the blade that have had metal removed to lighten the blade and the monolithic lightweight inserts are added to restore an aerodynamic shape to the blade that was altered by the inclusion of the cavity in the blade design. While both the lightweight inserts and the metallic portion of the blade are relatively monolithic materials having excellent strengths, the interface between the lightweight inserts and the metallic portion of the blade is the weak link wherin debonding and failure is most likely to occur. Typically, failures occur due to debonding between a primer applied to the metallic blade and the blade itself.
What is needed are improvements in the adhesion between the lightweight inserts molded into blade pockets and the metallic material of the blades at this interface. Pretreatment processes of lightweight jet aircraft fan blades that promote adhesion between the metal portion of the blade and lightweight inserts can provide such improvements.
The present invention is directed to improvements in the treatment of an aircraft engine fan blade comprised of lightweight inserts positioned in metal pockets. The fan blade is manufactured to have reduced weight by removing metal at preselected locations. These preselected locations take the form of pockets. The locations of the pockets are preselected so as not to adversely affect the structural integrity of the blade. The aerodynamic flow of air over the blade is restored by filling in the pockets with the lightweight inserts.
The bonding between the blade and the inserts is improved by a treatment method. First, the blade is pretreated by grit blasting, using a preselected grit to achieve a preselected surface finish, cleaning the surface, followed by a uniform etch in etchant followed by cleaning, prior to application of a primer. The etchant may be an alkaline solution, an acid solution, or a sequence that includes both alkaline and acid solutions, provided that cleaning and neutralization occurs between successive treatments in solutions. The lightweight insert material is applied to the pocket after the pocket is coated with a primer.
The advantage of the present invention is that the problem of poor bonding between the primer coating on the surface of the blade pockets and the metal that comprises the surface of the blade pockets is substantially reduced. This results in longer life of the fan blade and smooth airflow over the blade without out of balance conditions that can arise with loss of inserts. The present invention is a surface pretreatment prior to applying primer to the surface of the blade pockets. The increase in the bond strength between the primer and the surface of the blade pockets creates better adhesion between the primer and the metal surface of the blade, aid prevents delamination or loss of the inserts from the pockets.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.